At first I thought it might be a security issue or a problem with absolute vs. relative links, but it turned out to be caused by overlapping clickboxes. She had started the project by importing a Powerpoint file, selecting "Advance slide on click" rather than "Automatic advance" when importing. This setting creates large clickboxes that cover the entire slide, and the slide-encompassing clickbox was overlapping our link clickboxes.

We removed the slide-covering clickbox and added a Next button, which made more sense anyway, and the links now work.

One other thing to check, particularly if you are new to Captivate. When a clickbox is inserted, it has a duration. Check the Timeline to see that its duration lasts the whole slide duration, or at least until you will want to click it.